I live and work in the UK and my training is in Computer Science and Mathematics. I defended my PhD thesis eight years ago and since then gained work as an engineer, then a Post-doc Research Assistant, then a Teacher, and now a Research Associate. Is it fine for me to apply for another post-doc research assistant position, or is that not the done thing? I ask because in job progression terms the job title may seem to some University employers like "going backwards". I'm slightly bohemian when it comes to research positions - I'm not really concerned about fluctuations in pay, but rather whether the research is interesting.
There's no prohibition in most job postings against people like you applying for postdoc positions. In many cases, it will be rare, though, that someone would be selected who is 8 years past their PhD simply because most people recognize that postdoc positions are supposed to be training positions for people on their way to faculty jobs. That doesn't seem to be your trajectory, and so departments would probably pass you over. As they are training positions, postdoc positions are typically also strictly limited to 3 years which may or may not interest you.
The situation may be different if a postdoc position is paid for from grant money. In that case, the principal investigator may decide that to heck with training, I need to get this work done and this is the right person for it.
I don't think that you're doing yourself a disfavor if you apply. If it's a small enough community where people talk about each other, you may start to acquire a reputation as a bohemian without a clear career trajectory, but you already know that and so no harm done.